Level of organic maturity (“LOM”) is a parameter used in the assessment of the commercial value of a shale reservoir. LOM impacts interstitial fluids, production limits, pore pressure, and reservoir quality. One indicator of LOM is vitrinite reflectance, which is measured in a laboratory.
The presence of pyrite, graphite, and turbostatic carbon nanostructures (“TCN”) produces interfacial polarization that results in frequency-based dispersion of effective electrical properties of shale formations. Locating a concurrence of favorable geologic parameters such as thermal history, gas content, reservoir thickness, fractures, rock composition, total organic carbon (“TOC”), porosity, permeability, free and sorbed gases, water saturations, and thermal maturity assists in finding gas shale reservoirs. Gas is stored interstitially within pore spaces between rock grains, fractures, or kerogen, or it can be adsorbed to the surface of organic components contained within the shale. Kerogen is categorized into four broad groups, each of which has distinct bearing on what type of hydrocarbons will be produced. Type 2 kerogen is generated from the remains of planktons in reducing environments found in moderately deep marine settings. Sulfur is associated as pyrite, free sulfur, or in the organic structure of kerogen. Type 3 kerogen is primarily derived from terrestrial plant debris that has been deposited in shallow to deep marine or non-marine environments. Type 3 kerogen may generate dry gas, as it has lower hydrogen and oxygen content. Marine shale facies contain type 2 and type 3 blends. Heat and time convert organic material to hydrocarbons. Temperature and pressure increase during burial, resulting in organic material giving off oil and gas. Diagenesis refers to the low-temperature alteration of organic material due to oxidation, chemical processes, and biological processes. With increases in temperature and changes in pH, organic material is converted into kerogen and lesser amounts of bitumen. Pyritization occurs during the diagenesis phase when sulfate-reducing bacterial colonies sulfate in sea water to biodegrade organic material. Hydrogen sulfide and native sulfur released by these bacteria combine with iron in clays to form pyrites (FeS2). Catagenesis causes chemical bonds to break down in shale and the kerogen. Oil is produced from type 1 kerogen, waxy oil is produced from type 2 kerogen, and gas is produced from type 3 kerogen. Pyrite and marcasite are two sulfides in iron-rich sedimentary rocks: black shale and coal, respectively. Many shale reservoirs contain up to 10 wt % of pyrite and TOC that amount up to 7% pyrite and up to 20% kerogen by volume.
Shale formations may contain 12% to 15% kerogen volume. Spectral elemental analysis using geochemical logs is used to solve for the elements present in the formation. However, the volume of kerogen cannot be determined with spectral tools due to the presence of carbon in various other minerals. The Passey method is designed for use in the evaluation of organic content of hydrocarbon source rocks. The method uses knowledge of the maturity of the organic material, and is less accurate for sediments that are overmature, such as shale gas formations. Frequently, layers including a higher organic content are interspersed with layers of lower organic content or conductive minerals resulting in dielectric anisotropy that adversely affects the resistivity interpretation. Therefore, laboratory measurements of thermal maturation is another method and includes measurements of thermal indicators such as vitrinite reflectivity, LOM, spore color index, conodont alteration, sulfur content, hydrogen index, and pyrolysis byproducts. Vitrinite is a good indicator of the maximum temperature that a shale has been exposed to but some shales do not contain vitrinite.
Electrical conductivity values in the induction tool frequency range, and permittivity values in the propagation tool frequency range, are estimated using existing resistivity interpretation techniques to primarily evaluate water saturation in a conventional geological sample or the geological formation. The effects of pyrite, graphite, and other constituent minerals are not included in the resistivity interpretation for shale gas characterization. Source rocks are commonly shales and lime-mudstones that contain large amounts of organic matter. Richness and maturity of source rocks is evaluated through a variety of laboratory analyses like TOC analysis, pyrolysis, elemental analysis, vitrinite reflectance, thermal alteration index, gas chromatography, and visual kerogen description.